Portable devices like cellular phones, notebook computers, personal digital assistants (PDA), global position systems (GPS), cameras, camcorders, and music players are becoming more and more popular globally. Due to their relatively high price, high popularity and small size, it is more and more important to track such kinds of devices to make sure that they are within a safe range of their owners.
It is possible that these devices may be left on the table due to the incaution of their owners, for example, leaving the device in a restaurant or public restroom. This creates not only economic loss but also intellectual loss. Intellectual loss is incurred since important personal information and documents stored in these portable devices may also be lost at the same time without any suitable means for recovering these privacy records.
Moreover, as the sizes of portable devices become smaller and smaller, these devices are more likely to be put into a fashionable purse or pocket. As a result, the indication sound of some instant event, for example, a call ring for a cellular phone, or a scheduled alarm of PDA, is attenuated. The attenuated voice may be too low to be heard in a noisy environment. For those events that need an instant response, a missed call precludes taking advantage of some business opportunity or the handling of personal affairs in a timely fashion.